The present invention relates to an optical glass, composed of a basic composition of B2O3xe2x80x94RO (where R indicates Mg, Ca, Sr and Ba), having a refractive index (nd) of 1.52 to 1.65, an Abbe number (vd) of 55 to 61, and an 80% transmission wavelength (at a thickness of 10 mm including no reflection loss) of 330 nm or shorter; and a projection exposure apparatus using the same.
Recently, as optical systems have come into wider use, opportunities to design optical glasses using wavelengths in ultraviolet regions have been increasing. Therefore, demands for developing optical glasses having various optical constants and exhibiting an excellent transmissivity in ultraviolet regions have been becoming greater year by year.
A number of optical glasses having optical constants with a refractive index (nd) within the range of 1.52 to 1.65 and an Abbe number (vd) within the range of 55 to 61 have conventionally been known, and optical glasses represented by an SiO2xe2x80x94B2O3xe2x80x94Al2O3xe2x80x94BaO system have already been manufactured widely.
For example, Japanese Patent Application Laid-Open No. HEI 11-79781 discloses an optical glass of an SiO2xe2x80x94B2O3xe2x80x94Al2O3xe2x80x94La2O3xe2x80x94CaOxe2x80x94Li2O system as an optical glass having the above-mentioned optical constants and exhibiting an excellent chemical durability. Also, Japanese Patent Application Laid-Open No. SHO 62-87433 discloses an optical glass of an SiO2xe2x80x94B2O3xe2x80x94BaO system having an 80% transmission wavelength (at a thickness of 10 mm including no reflection loss) of 340 nm or shorter as an optical glass having the above-mentioned optical constants and exhibiting a high transmissivity in ultraviolet regions. Further, Japanese Patent Application Laid-Open No. HEI 03-93644 discloses an optical glass of an SiO2xe2x80x94B2O3xe2x80x94BaO system having an 80% transmission wavelength (at a thickness of 10 mm including no reflection loss) of 320 nm or shorter. In this specification, xe2x80x9c80% transmission wavelength (at a thickness of 10 mm including no reflection loss)xe2x80x9d is the wavelength measured according to JOGIS 17-1982 xe2x80x9cMeasuring Method for Internal Transmittance of Optical Glass.xe2x80x9d Specifically, a sample having a thickness of 10 mm is irradiated with light, the spectral transmissivity of the sample at the time when it includes no reflection loss is measured, and the wavelength (nm) of the irradiation light when the resulting spectral transmissivity is 80% (when 80% of all the light beams irradiating the sample are transmittable) is taken as the above-mentioned wavelength.
However, SiO2, which is introduced in the above-mentioned conventional optical glass in order to secure the chemical durability thereof and so forth, raises the temperature required for melting the glass. Therefore, impurities are more likely to mingle into the optical glass from crucible materials, furnace materials, and the like in the process of manufacturing the optical glass, whereby the lowering of ultraviolet transmissivity in the resulting optical glass has become problematic. Also, La2O3, introduced into the above-mentioned conventional optical glass in order to raise the refractive index thereof, lowers the ultraviolet transmissivity of the optical glass when introduced by a large amount.
In view of various problems of the above-mentioned conventional optical glasses, it is an object of the present invention to provide an optical glass having a refractive index (nd) of 1.52 to 1.65, an Abbe number (vd) of 55 to 61, and an 80% transmission wavelength (at a thickness of 10 mm including no reflection loss) of 330 nm or shorter and exhibiting an excellent i-line transmissivity; and a projection exposure apparatus using the same, which can attain a high resolution.
The inventors repeated diligent studies in order to achieve the above-mentioned object and, as a result, have found that an optical glass having predetermined optical constants mentioned above and exhibiting a high ultraviolet transmissivity is obtained in an optical glass of a B2O3xe2x80x94RO system (where R indicates Mg, Ca, Sr and Ba) if the composition ratio of constituents is adjusted into a specific range, thereby reaching the present invention.
Namely, the optical glass of the present invention comprises a composition in terms of weight ratio within the range of 0 to 14.8 wt % of SiO2, 30 to 60 wt % of B2O3, 0 to 17.5 wt % of Al2O3, 0 to 0.4 wt % of Li2O, 0 to 3 wt % of Na2O, 0 to 3 wt % of K2O, 0 to 5 wt % of MgO, 0 to 30 wt % of CaO, 0 to 30 wt % of SrO, 0 to 35 wt % of BaO, where RO (MgO+CaO+SrO+BaO) is 8 to 35 wt %, 0 to 20 wt % of ZnO, 0 to 6.5 wt % of La2O3, 0 to 15 wt % of Gd2O3, 0 to 15 wt % of Y2O3, 0 to 10 wt % of Ta2O5, 0 to 9.5 wt % of ZrO2, 0 to 1 wt % of As2O3, and 0 to 0.5 wt % of Sb2O3; and has a refractive index (nd) of 1.52 to 1.65, an Abbe number (vd) of 55 to 61, and an 80% transmission wavelength (at a thickness of 10 mm including no reflection loss) of 330 nm or shorter.
The optical glass comprising such a composition of constituents has a refractive index (nd) of 1.52 to 1.65, an Abbe number (vd) of 55 to 61, and an 80% transmission wavelength (at a thickness of 10 mm including no reflection loss) of 330 nm or shorter and exhibits an excellent i-line transmissivity.
The projection exposure apparatus of the present invention comprises an exposure light source, a reticle formed with a pattern original image, an illumination optical system for illuminating the reticle with light outputted from the exposure light source, a projection optical system for projecting onto a photosensitive substrate a pattern image outputted from the reticle, and an alignment system for positioning the reticle and the photosensitive substrate with respect to each other; wherein at least a part of optical glasses constituting the illumination optical system, optical glasses constituting the projection optical system, and the reticle is made of the optical glass of the present invention.
Since the projection exposure apparatus of the present invention comprises the optical system constituted by the optical glass of the present invention, it can yield an excellent resolution.